The way rape and sexual assault reports are looked into by the Metropolitan Police is to be investigated following allegations that officers have fiddled crime figures to meet targets.

The force's commissioner, Sir Bernard Hogan-Howe, told a parliamentary committee he was shocked by the claims.

A force whistleblower has said sexual offences are being under-reported by as much as a quarter.

Other police forces were also said to have manipulated their crime figures.

The allegations were made during a hearing of the Public Administration Select Committee in the Commons last month.

Review of cases

MPs were told by a variety of witnesses, including retired officers, that some victims - often those of sexual crimes - were encouraged not to file a complaint, resulting in a "no crime" recording.

Metropolitan Police constable James Patrick - who is currently awaiting disciplinary proceedings - told the committee his concerns had begun after he joined the force in 2009.

Analysing 12 months of data, PC Patrick said he found "the Met had effectively been under-recording rape and serious sexual offences by between 22% and 25%".

PC Patrick said he had learnt that, in an effort to avoid the perception of serious sex crimes going undetected, "a preference had developed to try to justify 'no crime' on the basis of mental health or similar issues of vulnerability or by saying that the victim has refused to disclose to them".

A former West Midlands chief inspector also described practices such as recording thefts as "lost property".

In response, Sir Bernard told a separate parliamentary committee on Tuesday that the claims were being taken very seriously and there would be a full investigation.

He said that would include an internal element, an external audit of crime figures conducted by staff from the Deputy Mayor's office, and a review of recent historical cases.

He said: "What we are going to have done is carry out a separate piece of research by some academics to go back and talk to women over at least the last two years where there has been 'no crime' to establish [whether] was that true?

"And if so, or if it wasn't true, did any police officer play a part in putting pressure on you as a person or anyone to withdraw that crime?"